MIAMI -- There will be no "hanging chads" this time around in Florida. The punch-card voting that plagued the 2000 presidential election in the state is long gone.

But with Florida's primary on Tuesday, some in the state are bracing for more potential ballot trouble because the new electronic touch-screen machines in much of the state have aroused doubts of their own.

Florida legislators voted essentially to ban them earlier this year, after confusion in a 2006 congressional contest in Sarasota wound up in court. But the next set of machines will not be ready until the general election in November, forcing election officials to press the controversial machines back into use one more time.

"Every place in Florida with the touch screens should be doing the election supervisors' prayer -- 'Oh, please, God, don't let this election be close,' " said Ion Sancho, election supervisor in Leon County and a longtime critic of the touch-screen machines. "They simply don't work well enough."

The lingering doubts over Florida's machines in part reflect the difficulties of ballot reforms instigated across the country in the wake of the disputed 2000 election. What officials found is that there is no perfect system for counting votes, and the switch to more modern means has not guaranteed reliability.

After Florida's presidential debacle in 2000, in which the outcome of the race came to depend on improvised rules about which marks on a punch card counted as a vote, Congress passed the Help America Vote Act in 2002. The law called for the phasing-out of punch-card systems.

The legislation was celebrated as reform, but its flaws soon became apparent here.

"When HAVA pushed all these counties to improve voting systems, people just didn't realize the problems that computers can have," said Alec Yasinsac, a professor of computer science at Florida State University who led part of the state investigation into the Sarasota machines. "It shows what can happen when you try to do a good thing but try to do it too fast."

Florida counties spent millions for new equipment after the 2000 election.

Some bought "optical scan" machines that read voters' marks on paper ballots -- similar to standardized tests and lottery tickets.

But Florida's most populous counties bought the touch-screen systems, which are familiar to anyone who has used an automated teller machine. They were modern but offered no "paper trail" -- in other words, no other record of what voters intended besides the digitized computer records.

It is that lack of a paper trail that contributed to the furor after the 2006 race for the 13th Congressional District. Christine Jennings (D) faced Vern Buchanan (R) for the seat vacated by Katherine Harris, who gained fame for her role presiding over the state's electoral apparatus during the much-disputed 2000 contest.

The new touch-screen machines were in place in Sarasota for the election, and they were assumed to be an improvement. But after the polls closed, a troubling anomaly appeared in the results.

There were 18,000 "undervotes" in Sarasota County -- that is, 18,000 people showed up to the polls and chose candidates in other contests but not in the prominent and hard-fought congressional race. In the four other counties where voters cast ballots in the same race, the undervote percentage was far smaller.

What happened?

The undervotes, some speculated, could have been the result of a confusing ballot design that led voters to skip over the race. Others guessed that the negative tone of the campaign led voters to skip it.

But some suspected that the machines dropped the votes, and a scattering of voters claimed the machines had given them trouble.

Exactly what happened has not been resolved.

None of the official investigations found a bug in the machines that would have caused the dropped votes.

But that, some of the same investigators say, doesn't mean there wasn't a flaw.

In October, the U.S. Government Accountability Office summarized the state of the investigations and research into what happened this way:

"We found that some of the prior tests and reviews provide assurance that the voting systems in Sarasota County functioned correctly, but they are not enough to provide reasonable assurance that the [machines] did not contribute to the undervote."

It also warned that even with more tests, there will never be certainty about what happened: "Absolute assurance is impossible to achieve."

If the machines had left a paper trail of each voter's actions -- such as the punch cards or the lottery-ticket ballots -- many believe auditors would have had important clues to what happened.

"The point is, these [touch-screen] machines don't maintain any records that would allow you to see whether voters were having difficulties," said Douglas W. Jones, a professor of computer science at the University of Iowa who has studied the machines. "The race in Florida's 13th Congressional District was a perfect example of that."

In the wake of the investigations and ambiguity, the Florida legislature moved earlier this year to switch to voting machines that leave a paper trail.

In places such as Miami-Dade and Broward counties, which spent $24.5 million and $17 million, respectively, for the touch-screen machines, that means having to shell out again for new machines. With the exception of Sarasota County, the counties switching machines will not be ready with the technology until the next election.

For now, election supervisors are expressing confidence in the machines they are about to abandon.

"We don't have any reason to believe that they're not reliable," said Sterling E. Ivey, spokesman for the state elections officials.

But if they're reliable, why did the legislature move to get rid of them? "Floridians have said they want to be able to cast a ballot on a piece of paper," Ivey said. "We're moving to a paper system to help restore confidence."